(a) Field of the Invention
The invention consists in treating wood to permanently reduce its hardness. More particularly, the present invention relates to a treatment for providing a permanent decrease of the hardness of wood, and an increased moisture content at equilibrium by in situ polymerization of prepolymers therein. The invention is also directed to articles of wood which are impregnated with polymeric networks providing a permanent reduction of their hardness, and an increased moisture content at equilibrium.
(b) Description of Prior Art
Contrary to oil-borne formulations, wood preservatives formulations based on water soluble chemicals cause wood to harden and check, making wooden posts more difficult to climb. The most common waterborne preservatives based on copper and chromium are copper-chromium (CC), copper-chromium-arsenate (CCA), copper-chromium-boron (CCB), copper-chromium-fluoride (CFK), copper-chromium-fluoroborate (CCFB) and copper-chromium-phosphorous (CCP). Other waterborne preservatives exist that include copper or chromium. The most common formulations are ammoniacal copper arsenate (ACA), ammoniacal copper zinc arsenate (ACZA), chromated zinc chloride (CZC) and copper-quaternary ammonium compound (ACQ). A CCA type C solution normally contains 19% CuO, between 16 and 45% As.sub.2 O.sub.5 and between 36 and 65% CrO.sub.3. To these particular CCA formulations, various additives may be added which have the property of decreasing wood hardness to a level wherein its climbability is acceptable. These additives include polyethylene glycol (PEG 1000), marketed under the trade designation CCA-PEG, and polyethylene glycol of higher molecular weight (PEG 8000), marketed under the trade designation CCA-PEG.sup.+. Another additive is a wax which makes wood impermeable and which has been marketed under the trade designation CCA-ULTRAWOOD.RTM.. Finally, an additive based on a formulation containing an oil emulsion in water is sold under the trade designation CCA-ET.TM..
Treatment of posts with CCA or equivalent waterborne preservatives is normally carried out under specific conditions described in the Standard C4-91 of the American Wood Preservers' Association (AWPA) or the equivalent Standard CAN/CSA-O80.4-M89 of the Canadian Standards Association (CSA) according to a so-called "full-cells" process.
Generally, the posts are introduced into an autoclave in which a vacuum (e.g. 636 mm Hg) is applied to remove the air contained in wood cells. Then, the treating solution of CCA at 2% is introduced into the autoclave and a maximum pressure of for example 1040 kPa is applied so as to enable the penetration of the preservative into wood. This pressure is maintained for about 5 hours for red pine and the solution is thereafter flushed away. CCA is then fixed to the wood at a temperature of for example 60.degree. C. for a few hours.
It has been shown by Gilbert et al. in Forest Products Journal, 47:3, (1997), pp 81-88, that formulations containing PEG 1000 are not permanent since an important fraction of the polyethylene glycol located in the first two centimeters of depth is washed away when posts treated therewith are in use. On the other hand, wax formulations cause the wood to harden since the latter dries by losing its natural humidity, which cannot be restored due to its surface which is made impermeable. Finally, although the treatment with an oil emulsion appears to ensure appropriate climbability characteristics, the oil is not bound to the wood and is therefore free to migrate out of the pole.
Japanese Laid-Open Application 1196302 published Aug. 8, 1989 discloses a method wherein a woody material is impregnated with an aqueous solution of water soluble and radically polymerizable compounds, water soluble and radically polymerizable metal salts of organic acids and possibly water soluble salts of inorganic acids. The radically polymerizable compounds may be glycerin di(meth)acrylate, trimethylolpropane tri(meth)acrylate, polyethylene glycol mono(meth)acrylate or polyethylene glycol di(meth)acrylate. The water soluble and radically polymerizable metal salts are preferably acrylates of Zn, Ba, Ca, Mg or Al. Wood thus treated has good durability, flame resistance, hardness, dimensional stability, rot resistance and insect resistance. This document however does not disclose a specific hardness which is maintained during the entire life of a pole.
Japanese Laid-Open Application 5220712 published Aug. 31, 1993 discloses treating wood material with a formulation comprising polyethylene glycol monoacrylate and/or polyethylene glycol monomethacrylate, a cross-linking agent and a polymerization catalyst. Wood material thus treated has improved durability and dimensional stability at a low impregnation degree. As in the previous Laid-Open Application nothing is mentioned about the production of a wood article having a permanent reduced hardness.
Japanese Patent 79012524 dated May 23, 1979 describes the manufacture of wood plastic composites using an ethylenically unsaturated monomer and a polymerization initiator, thereby preventing local heating and excess buildup. This patent is mute with respect to the hardness of a wood treated by that process, and is therefore not relevant to the subject matter of the present invention.
U.S. Pat. No. 4,567,115 issued Jan. 28, 1986 describes treating wood with an aqueous solution containing water, hexavalent chromium, copper, arsenic and polyethylene glycol 1000. The polyethylene glycol 1000 introduced into the wood treated by this process is easily washed away so that the reduced hardness obtained is not permanent.
Canadian Patent No. 1,187,255 issued Sep. 21, 1989 corresponds to U.S. Pat. No. 4,567,115.
Japanese Laid-Open Application 9160645 published Mar. 26, 1991 discloses the preparation of resin-impregnated wood for exterior walls by immersing wood in a polymerizable resin solution including a pyrolignous acid as a preservative. Contrary to the teaching of the present invention, the material obtained is hard (high strength) and does not undergo a permanent reduced hardness.
U.S. Pat. No. 5,098,589 issued Mar. 24, 1992 describes a polymer made of an alkylene oxide chain which can be used as an ion-conductive conductive polymer electrolyte. There is no teaching whatsoever that the polymer may be used to reduce the hardness of wood.
U.S. Pat. No. 3,713,943 issued Jan. 30, 1973 describes a process for treating wood with a polyethylene adduct of a compound containing multi active-hydrogen functionality. This treatment improves the bondability of the wood by reducing the criticality of drying conditions. This has nothing to do with hardness reduction in wood.
Canadian Patent No. 907,232 issued Aug. 8, 1972 relates to the preservation of wood using a monomeric or polymeric methylacrylate which is used in association with a preservative, in order to prolong the service life of wood products, and has nothing to do with the reduction of wood hardness.
Canadian Patent No. 866,158 issued Mar. 16, 1971 describes a method of manufacturing hard, synthetic polymer-containing wood wherein wood is impregnated with a mixture of a monomer, an additional polymer and a free radical catalyst. The method gives a hard product contrary to the aim of the present invention, which instead intends to reduce its hardness.
U.S. Pat. No. 4,205,097 issued May 27, 1980 relates to a process for producing an impregnated article by impregnating it with a polymerizable solution followed by heat treatment, so as to give a product which needs no after-treatment. Again this treatment has nothing to do with reduced hardness of the product obtained.
Swedish Patent 9002302 issued Jun. 29, 1990 relates to a wood treatment with a multi functional allyl ether, a (meth)acrylate terminated polyether and/or urethane prepolymer to provide a wood which is dimensionally stable. This invention is of course totally unrelated to the aim of the present invention which intends at permanently reducing the hardness of wood.
Japanese Laid-Open Application 4259506 published Sep. 16, 1992 discloses the use of a dicarboxylic anhydride with a methacrylate to impregnate wood in order to give it high dimensional stability and weather resistance. The same comments apply as those made with respect to Swedish Patent 9002302.
Other references of interest include French Application 2,661,685 published Nov. 8, 1991, Canadian Application No. 2,041,561 published Nov. 2, 1991 and French Application 2,278,708 published Feb. 13, 1978.
It will be seen that none of the references mentioned above provides for a method or a product wherein hardness is substantially permanently reduced, as well as an increased moisture content at equilibrium.
There is thus a need for a treatment with an additive which enables to permanently reduce wood hardness.